Fusion research apparatus



July 13, 1965 I p. w. KERST ETAL. v 3,1

FUSION RESEARCH APPARATUS paged Aug. 20, 1962 w 2 Sheets-Sheet "-1 July13, 1965 D. w. KERST ETAL FUSION RESEARCH APPARATUS 2 Sheets-Sheet 2.

Filed Aug. 20, 1962 United States Patent 3,194,739 FUSION RESEARCHAPPARATUS Donald W. Kerst, Madison, Wis, and Tihiro Ohkawa,

San Diego, Calif., assignors to General Dynamics Gorporation, New York,N.Y., a corporation of Delaware Filed Aug. 2-0, 1962, Ser. No. 217,959 9Claims. (Cl. 176-1) The present invention relates to fusion researchapparatus and more particularly to an improved apparatus for heating andconfining particles for such research.

In prior artfusion research apparatus, magnetic fields have beenemployed to confine plasma. The magnetic field has been set up bypassing a high current through a plasma disposed in a discharge tube,which current produces a concentric magnetic field which confines andcompresses the plasma. Plasma has been found to be highly unstable insuch apparatus. Attempts have been made to stabilize the plasma byadding a magnetic field which extends parallel to the discharge in thetube. This additional stabilizing field has proven to be fairlyeffective and in some cases the plasma may be stable. However, becauseof the concavity of the fields with respect to the plasma, the plasmahas been difficult to stabilize. The stability of the plasma isdependent critically on the influence of the plasma on the magneticfield distribution. Moreover, the confined plasma has been extremelysensitive to magnetic field errors.

An alternate method for confining hot plasma is the so-called cuspconfiguration of magnetic fields. In such a geometry, plasma is injectedinto a central volume defined by at least a pair of loops of wires orcoils. Oppositely directed currents are passed through adjacent coils tothereby provide a series of convex magnetic fields (i.e., convex towardthe plasma).

Such a geometry provides a magnetic field which increases with radiusfrom the axis of symmetry. In other words, the equilibrium position ofplasma is at the center of the cusped geometry. Perturbations of theplasma in this equilibrium position protrude. into regions of strongermagnetic fields which force the perturbations back to the equilibriumposition. Even individual particles escaping from the main body ofplasma find themselves in a stronger magnetic field from which they areejected back into the body of the plasma.

The most serious drawback of the cusp confinement geometry has been thehigh particle loss rate at the ring of the cusp or at the point of thecusp. The lines of force leading out of the confining region carryparticles outwith their ion thermal speed, and thus the leakage at thering cusp and point cusp is at a very high rate. It has been suggestedthat the leakage at the ring cusp may be eliminated by connectingadjacent ring cusps together. In one such proposed apparatus currentcarrying rings are disposed inside a solenoid having a longitudinalmagnetic field. Such a geometry may eliminate the losses in the ringcusps but the proposed apparatus is unstable and complicated.

An object of the present invention is the provision of an improvedplasma research apparatus. Another object plasma research apparatusemploying various features of the present invention, and with portionsthereof being broken away to show certain features thereof;

FIGURE 2 is an enlarged fragmentary perspective view as viewed alongline 22 of FIGURE 1;

FIGURE 3 is an enlarged diagrammatic view of a portion of FIGURE 2; and

FIGURE 4 is an enlarged diagrammatic view of a portion of FIGURE 3showing how plasma may be trapped in the apparatus.

Generally, as shown in the drawings, a plasma research apparatusincludes means 10 for providing alternate interconnected convex andconcave magnetic confining fields about an area 12 Where plasma is to beconfined. Additional means 14 is included in the apparatus in eachconcave field for providingan additional convex magnetic field betweeneach pair of the first mentioned convex fields. The total magnetic fieldstrength in the space between the additional means and the portion ofsaid first mentioned means which provides the concave field is madesubstantially greater than that of the first mentioned convex magneticfield to thereby stabilize the plasma.

More specifically, in the illustrated embodiment, a toroidalconstruction is shown for confining plasma. In the structure illustratedin FIGURE 1, convex fields are provided around the minor circumferenceof the toroid by a plurality of endless rods or members 14 of conductivematerial, such as copper. A current is induced in each of the rods by apower source 15, as hereinafter described in detail. The currentestablishes a magnetic field about each rod 14, the fields being convexrelative to the minor axis of the toroid.

The alternate convex and concave magnetic confining fields are providedby a toroidal jacket 10 of conductive material such as copper. Thejacket 16 is internally fiuted or corrugated top rovide a plurality ofalternate longitudinally extending ridges 16 and grooves 18. One each ofthe rods 14 is received in each groove in spaced relation to the wallthereof. When a current passes longitudinally through each rod, currentis induced in the jacket which provides concave fields between the rodsand the wall of the grooves 18 and convex fields at the ridges 16.

In the illustrated embodiment, the rods 14- are supported centrallywithin the grooves 18 by a plurality of small supports 24 in the form ofretractable, pointed pins exof the invention is the provision of aplasma research tending through the jacket 10. The pins 24 are disposedso as to maintain the rods 14 in position against the force of gravitywhen the magnetic fields are not present in the torus. The pins 24, bymeans such as solenoids 2d, are quickly retracted at the same time asthe magnetic fields are established and prior to the injection of theplasma.

Once the magnetic fields are established the rods 14 are maintained inthe proper position by the magnetic field, the rods 14 and the grooves18, in the illustrated embodiment, being shaped to maintain the rods 14in an equilibrium position. In this connection, ,as shown particularlyin FIGURE 3, each rod 1 is of a generally ovalcrosssection. The smallerarcuate end portion 28 of the oval is faced inward or toward the minoraxis of the toroid and the larger arcuate end portion, 30 of the rod 14is faced outward or toward the wall of the groove 18. The oval endportions 28 and 3d of each rod 14 areintegrally connected by anintermediate portion 32 having generally fiat sides.

The jacket liladjacent the rod 14 is formed so as to generally conformto the shape of the rod 14. The gap 34 between the side portion 32 ofthe rod 14 and the jacket 10 is made slightly narrower than the gap .36between the enlarged end portion 39 of the rod 14 and the jacket 19. Inthis way, magnetic pressure at the fiat sides of the rod 14 is directedpartially back into the groove 18 to thereby compensate for the magneticpressure applied by the field in the region between the enlarged endportion 39 of the rod 14 and the wall of the groove 13, which pressuretends to force the rod 14 out of the groove To obtain an equilibriumposition, the dimensions of the rod 14 and groove 18 are preferablyselected so as to satisfy the following:

B22 X D ZBGZZ where, as shown in FIGURE 3 l is the projection of thestraight sides of the rod 14 on the diameter of the large end 3 of therod 14,

B is the strength of the magnetic field in region between the rod 14 andthe wall of the groove 13.

D is the diameter of the large end 30 of the rod, and

B is the strength of the magnetic field in the region between the flatside of the rod 14 and the adjacent jacket wall It).

Other means may be employed to initially position the rods 14 in thegrooves 18 before the magnetic fields have een applied. For example,with no current passing through the rods 14 and no support, the earthsgravitational field causes each rod 14 to rest on the wall of thegrooves 1% immediately below the rod 14. The rods 14 may be theninitially positioned in the proper position in the groove 18 by a swiftdownward motion of the whole jacket for a short distance. The rods 14,because of inertia, move away from the downwardly moving jacket 14? and,at the same time, the magnetic fields are applied which then easilymaintain the rods 14 in position against the force of gravity.

As previously indicated, current is induced in each rod 14 and, in turn,in the jack t 10 by a power source 15. As shown in FIGURE 1, the powersource includes a torodial core transformer 38 which is disposed so thatthe rods 14 pass through the center hole or major axis thereof, andthereby act as short circuited secondary windings of the transformer.The transformer 38 includes a core 40 of iron laminations, and a primarywinding or coil 42 of wire wrapped about the core 40.

As shown in FIGURE 1, the transformer is provided with a toroidalhousing 44 of conductive material, such as copper, which is disposedabout the primary winding 42. An annular gap 46 is provided in thehousing adjacent the major axis thereof. The housing 44 is connected toa transverse gap 47 in the jacket 10 by a pair of spaced apart flanges48 of conductive material, such as copper, which extend between the endsof the jacket 1d and the ends of the housing 44. The primary winding 42of the transformer 38 is connected to a source of pulsatory power (notshown) which may be of the conventional type, such as a mercury tube anda capacitor bank.

Because of the toroidal shape, the rods 14 have different lengths ormajor circumferences. When a current is passed through the primarywinding 42, each rod 14, because it is a shorted turn, generates thesame magnetic flux. Therefore, the current density required in thejacket wall It around the short rods 14 is higher than the currentdensity required in the jacket wall it) around the longer rods. If theproper current density is not available to locally match the currentdensity required, a transverse magnetic field will be set up at the gap47 between the ends of the jacket 10. Therefore, the turns of theprimary winding 42 are preferably distributed on the core 46 so that theflow pattern of the flange surface currents goes generally directly intoward the grooves 13 and does not cross the whole flange 48 to thegrooves 18 surrounding the shorter rods 14.

Preferably, to further insure proper distribution of the current, thegap between the flanges 48 is tapered toward the major axis of the torusIt) so that the Width of the gap is inversely proportional to thedistance of the point on the flange 43 from the major axis of the torus.Moreover, perforations 50 are preferably provided in the flanges 48 toadditionally insure the proper distribution of current flow lines on thesurface of the flanges 48.

So that the trapped plasma does not exchange its charge with neutralgas, and thereby cause hot ions to be neutralized and to escape acrossthe magnetic confining fields, the plasma region 12 is maintained at ashigh a vacuum as possible (i.e., as free as possible from neutral atomsand molecules). In the illustrated embodiment means 52 are provided forrapidly evacuating the plasma region. in this connection, a substantialnumber of apertures 54 are provided in the jacket it and a manifold 56in the form of a generally rectangular toroidal can of conductivematerial is disposed around the jacket 10, the jacket being suitablysupported and insulated therefrom. The manifold 56 is also suitablyinsulated from the housing 44. A suitable vacuum pump (not shown) isconnected to the manifold 55.

As shown particularly in FIGURE 2, the plasma is injected into thejacket 10 by a source 53 of plasma, such as a conventional plasma gun,extending through the manifold 5t; and the wall of the jacket 16.Preferably, a plasma gun is employed in which the neutral gases aretrapped and not permitted to enter the plasma region. \Vhile only onegun is shown, a sufficient number of guns are arranged about the jacketto provide the desired plasma density.

The plasma which is injected into the magnetic confining field adjacentone of the rods 14 is trapped by discharging the polarization thereof.In this connection, as shown particularly in FIGURE 4, the injectedplasma crosses a magnetic line of force established by the rod 14 at Mand progresses to the same line of force looped back at N. For a plasmavolume to cross a line of force, a polarization charge must be creatednear the surface of the plasma volume. The surface charge of the plasmavolume must reverse in polarity or else the plasma volume cannot crossthe line of force looped back in the opposite direction. This reversepolarity cannot be established on the looped back line of force andhence the plasma comes to rest and is trapped in the system. Morespecifically, the reverse polarity cannot be established because theelectrons are free to travel along the looped back line of force andthereby short-circuit the polarities called for at M with the reversepolarities called for at N. Rather than short-circuiting the plasma byemploying a line of force looped back into the plasma path, anoppositely directed plasma may be employed to short-circuit thepolarization fields.

Other methods may be employed to trap the particles within the plasmaregion. For example, the plasma may be injected at one of the ridges.While the injected plasma is pushing on the concave side of the ridgefield, an unstable configuration exists and the injected plasma punchesthrough into the plasma region. However, once inside, plasma attemptingto reverse its direction and to emerge through the now convex lines offorce, encounters a stable configuration, thereby finding escapeimpossible.

In order for the plasma to be stable in the apparatus described above,the magnetic field strength B in the region between the rod 14 and itsgroove wall (groove region) is made substantially greater than themagnetic field strength B established in the region adjacent the ridge16. In order to avoid the worst perturbations that could cause trouble,the dimensions of the structure are selected so that:

where, as shown in FIGURE 3,

d=half the gap between the jacket wall and the rod,

r is the radius of the groove, less half the gap (d),

and

(p is the angle around the rod in which the lines of force are curvingaround the rod and not away from the same.

In high order multipoles (i.e., sixpole fields or greater) 5 isapproximately equal to 1r, and hence the square of the ratio of themagnetic field strength in the ridge region to that in the groove ismade less than the ratio of the half gap to the radius of the groove 18.

In one embodiment of the plasma research apparatus, a copper jackethaving a 3 cm. wall, is formed so as to provide six ridges and sixgrooves. The wall is perforated so that approximately 25 percent of itsarea is open and the jacket is disposed within a copper manifold. Theridges are made with a radius of .15 cm., and the grooves are made witha radius of 28 cm. Six oval shaped rods are disposed in the grooves. Theradius of the larger end of each rod is 22 cm., and the radius of thesmaller end of each rod is 15 cm. The projection of the fiat side of therods on the diameter is 7 cm., and a cm. gap is provided between theflat side of the rod and the jacket. The rods are supported in positionby retractable pins which are retracted in approximately 0.1

sec.

The capacitor bank is charged to a voltage such that a flux density of15,000 gausses is in the core of the transformer, and the length of thecurrent pulse is 0.1 second. The plasma region is maintained at a vacuumof mm. of Hg. A sufiicient number of plasma guns are connected to thejacket to provide a plasma density of 10 particles per cm.

It should be realized that while the structure shown and described aboveis a toroidal construction, certain features of this invention may beemployed in a linear type of plasma research device. Moreover, insteadof having the current induced in the rods, the rods may be separatelyexcited by parallel leads extending through a wall of the jacket. Localmagnetic fields generated about .the parallel leads guard or protect theleads from the hot plasma. 7 7

Various other changes and modifications may be made in the abovedescribed plasma research apparatus without deviating from the spirit orscope of the present invention. 7

Various features of the present invention are set forth in theaccompanying claims.

We claim:

1. Apparatus for confining plasma comprising means for providingalternate interconnected convex and concave magnetic confining fieldsabout an area where the plasma is to be confined, and additional meansin each of the concave fields for providing an additional convexmagnetic field between each pair of the first mentioned convex fieldswhereby the plasma area is encircled by a series of convex fields, thetotal magnetic field strength in the space defined between theadditional means and the portion of said first mentioned means whichprovides the concave field being made substantially greater than that ofthe first mentioned convex magnetic field to thereby stabilize theplasma.

2. Apparatus for confining plasma comprising means for providingalternate interconnected convex and concave magnetic confining fieldsabout an area where the plasma is to be confined, and additional meansin each of the concave fields for providing an additional convexmagnetic field between each pair of the first mentioned convex fieldswhereby the area is encircled by a series of convex fields, the squareof the ratio of the total magnetic field strength in the region betweenthe additional means and the concave field providing portion of saidfirst mentioned means to the first mentioned convex magnetic fieldstrength being less than the ratio of half the thickness of the regionto the radius of the concave field providing portion.

3. Apparatus for confining plasma comprising an elongated, generallytubular jacket of conductive material, said jacket being internallyfluted to provide a plurality of generally longitudinally extending,generally arcuate ridges and grooves, an elongated conductive memberdisposed in each groove in spaced relation to the wall thereof, theportion of said conductive member adjacent the wall of said groove beinggenerally arcuate in shape, and means causing a current to pass throughsaid members and said jacket thereby establishing a first magnetic fieldin an area adjacent said ridges and a second field in an area defined bythe wall of each groove and the adjacent arcuate portion of theconducting member, the strength of the second magnetic field beingsubstantially stronger than that of the first magnetic field.

4. Apparatus for confining plasma comprising an elongated, generallytubular jacket of conductive material, said jacket being internallyfiuted to provide a plurality of generally longitudinally extending,generally arcuate grooves andridges, an elongated conductive memberdisposed in each groove in spaced relation to the wall thereof, theportion of said conductive member adjacent the Wall of said groove beinggenerally arcuate in shape, and means causing a current to pass throughsaid members and said jacket thereby establishing a first magnetic fieldin an area adjacent said ridges and a second field in an area defined bythe wall of each groove and the adjacent arcuate portion of theconducting member, the square of the ratio of strength of the firstmagnetic field to that of the second magnetic field being less than theratio of half the spacing between the arcuate portion and the groovewall to the radius of the groove wall.

5. Apparatus for confining plasma comprising an elongated, generallytubular jacket of conductive material, said jacket being internallyfluted to provide a plurality of generally longitudinally extending,generally arcuate grooves and ridges, an elongated conductive memberdisposed in each groove at a distance 2d from the wall of the groove,the portion of said conductive member adjacent the groove beinggenerally arcuate in shape, and means for causing currents to passthrough said members and said jacket thereby establishing a firstmagnetic field having a field strength B in an area adjacent said ridgesand a second field in which the lines of force curve around the arcuateportion in an area defined by the groove wall and the surface of thearcuate portion of the conductive member and which has a field strengthE the field strengths being defined by the relation r =the radius of thegroove wall minus d, and

=the angle around the arcuate portion in which the lines of force arecurving around the conducting member and not away from the same.

6. Apparatus for confining plasma comprising an elongated, generallytubular jacket of conductive material, said jacket being internallyfiuted to provide a plurality of generally longitudinally extending,generally arcuate grooves and ridges, an elongated conductive memberdisposed in each groove at a distance 2d from the wall of the groove,the portion of said conductive member adjacent the groove wall beinggenerally arcuate in shape, removable means on said jacket forsupporting said con ductive members in position when current is notpassing therethrough, and means for causing currents to pass throughsaid members and said jacket thereby establishing a first magnetic fieldhaving a field strength B in an area adjacent said ridges and a secondfield in which the lines of force curve around the arcuate portion in anarea defined by the groove wall and the surface of the arcuate portionof the conductive member and which has a field strength B said groovesand associated conductive members being shaped so that the magneticfields maintain the members in an equilibrium portion in the grooves,the field strengths being defined by the relation 1 z) 2) 2) where r=the radius of the groove wall minus d, and

=the angle around the arcuate portion in which the lines of force arecurving around the conducting member and not away from the same.

7. Apparatus for confining plasma comprising a toroidal jacket ofconductive material, said jacket being internally fluted to provide-aplurality of arcuate grooves and ridges which extend parallel to theminor axis of the jacket, a conductive member formed into a ring disposed in each groove at a distance 2d from the wall of the groove, theportion of said conductive member adjacent the groove wall being arcuatein shape, said jacket being provided with a transverse gap, and meanscoupled to said jacket at said gap for inducing a pulsatory current insaid member whereby a magnetic field of strength B in which the lines offorce curve around the arcuate portion is established in an area definedby the groove wall and the surface of the arcuate portion of theconductive member, said field inducing a current to flow through saidjacket thereby establishing a magnetic field having a field strength Bin an area adjacent said ridges, the field strengths being defined bythe relation r =the radius of the groove wall minus a', and

=the angle around the arcuate portion in which the lines of force arecurving around the conducting me: her and not away from the same.

8. Apparatus for confining plasma comprising a toroidal air tightchamber, means connected to said chamber for evacuating the same, atoroidal, perforated jacket of conductive material disposed in saidchamber, said jacket being internally fluted to provide a plurality ofarcuate grooves and ridges which extend parallel to the minor axis ofthe jacket, a conductive member formed into a ring disposed in eachgroove at a distance 2d from the wall of the groove, the portion of saidconductive member adjacent the groove being arcuate in shape, saidjacket being provided with a transverse gap, and means coupled to saidjacket at said gap for inducing current to flow in said members wherebya magnetic field of strength B in which the lines or" force curve aroundthe arcuate portion is established in an area defined by the groove walland the surface of the arcuate portion of the conductive member, saidfield inducing a current in said jacket thereby establishing a magneticfield having a field strength B in an area adjacent said ridges, thefield strengths being defined by the relation where r the radius of thegroove wall minus d, and

=the angle around the arcuate portion in which the lines of force arecurving around the conducting member and not away from the same.

9. Apparatus for confining plasma comprising an elongated, generallytubular jacket of conductive material, said jacket being internallyfiuted to provide a plurality of generally longitudinally extending ovalgrooves and arcuate ridges, the larger end of the oval being outermost,an elongated conductive oval shaped rod disposed in each channel withthe larger cross section outermost, the rod being spaced at a distance2d from the outermost wall portion of the core and at a slightly smallerdistance at the sides thereof, and means for causing currents to passthrough said rods and said jacket thereby establishing a first magneticfield having a field strength B in an area adjacent said ridges and asecond field having a field strength B in a region defined by the wallof each groove and the rod, the field strengths being defined by therelation 1 z) 2) M where r =the radius of the groove wall minus d, and

=the angle around the arcuate portion in which the lines of force arecurving around the conducting member and not away from the same.

References Cited by the Examiner UNITED STATES PATENTS 2,961,559 11/60Marshall 1'/68 3,009,080 11/61 Loos 1768 3,012,955 12/61 Kulsrud et al.1769 3,038,099 6/62 Baker et al. 315111 X 3,085,173 4/63 Gibson et al315111 X FOREIGN PATENTS 859,459 1/ 61 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

1. APPARATUS FOR CONFINING PLASMA COMPRISING MEANS FOR PROVIDINGALTERNATE INERCONNECTED CONVEX AND CONCAVE MAGNETIC CONFINING FIELDSABOUT AN AREA WHERE THE PLASMA IS TO BE CONFINED, AND ADDITIONAL MEANSIN EACH OF THE CONCAVE FIELDS FOR PROVIDING AND ADDITIONAL CONVEXMAGNETIC FIELD BETWEEN EACH PAIR OF THE FIRST MENTIONED CONVEX FIELDSWHEREBY THE PLASMA AREA IS ENCIRCLED BY A SERIES OF CONVEX FIELDS, THETOTAL MAGNETIC FIELD STRENGTH IN THE SPACE DEFINED BETWEEN THEADDITIONAL MEANS AND